The main objective of this proposed twin study is to use data from multiple informants, obtained at multiple time points with multiple assessment techniques to identify heritable phenotypes for future gene-mapping studies of ADHD and related behaviors. To accomplish this objective, the investigator will analyze standardized parental reports of Attention Problems, Aggressive Behavior, and Anxious/Depressed behavior syndromes, as measured by the Child Behavior Checklist (CBCL) which have been shown to be highly predictive of ADHD and its most common comorbid conditions. CBCLs have been collected on 3,084 twin pairs by the Netherlands Twin Registry when they were 3 and 7 years old. To these data, he will add parent (CBCL and Parent Rating Scale- CPRS) and teacher reports (Teacher's Report Form- TRF, and Conners' Teacher Rating Scale-CTRS) at ages 10 and 12. He will also add maternal DSM-IV interview data on a sub-sample of 250 at high risk for ADHD and 250 non-ADHD twin pairs. This design has the following strengths: 1) It combines features of a twin study with those of a longitudinal design; 2) It adds teachers' report data to a study that previously had only parental report data; 3) It adds Conners' data to a study that previously only had CBCL data; 4) It collects DSM-IV data via interviews on a selected sample of the twins who have been well described with CBCL data at ages 3, 7, 10, and 12 and with TRF/Conners' data at ages 10 and 12. With these data, the investigator will achieve the following aims: 1) Test informant variance effects by combining mother, father, and teacher ratings; 2) Measure instrument variance by combining CBCL, Conners' and DSM Interview data to assess genetic correlation for symptoms of ADHD and related behaviors across assessment approaches; 3) Measure developmental continuity and change in genetic and environmental contributions to symptoms of ADHD and related behaviors; 4) Test gender differences in the genetic and environmental contributions to symptoms of ADHD and related behaviors; and 5) Use genetic latent structure models to estimate genetic and environmental influences on comorbidity of symptoms of ADHD and related behaviors. Potential benefits of this research include the identification of genetically discriminating phenotypes for the study of ADHD and related disorders. By identifying these phenotypes, this research will help guide future molecular genetic studies of ADHD and related behaviors. A long-term goal of this project is to continue to study these subjects in order to extend the study of genetic factors from childhood, across adolescence, and into adulthood.